The present invention relates to ohmic resistors of the bulk resistance type having a large mass of semiconductor material. The method of fabrication of resistors of this type and especially silicon resistors forms part of the invention.
It is a current practice to produce rods of silicon doped right through by a p-type impurity and having an ohmic resistance which exhibits considerable variation with temperature. Thus the resistivity of material of this type is multiplied by about three within the temperature range of 20.degree. C. to 200.degree. C. It is in fact known that the resistivity of silicon is inversely proportional to the number of conduction holes, or in other words of free acceptor atoms, and to their mobility, in accordance with the formula: ##EQU1## where: q represents the charge of the electron,
.mu..sub.p is the mobility of the holes, PA1 N.sub.p is the number of conduction holes. PA1 T is the absolute temperature in degrees Kelvin and PA1 .alpha. is a suitable coefficient.
Within a range of -50.degree. C. to +200.degree. C., it can be considered that the number of conduction holes is substantially constant but that, on the other hand, their mobility varies in accordance with the formula: EQU .mu..sub.p =.alpha.T.sup.-2.2 ( 2)
where
It is deduced from formulae (1) and (2) that, within the temperature range indicated, the resistivity is approximately proportional to the power 2.2 of the absolute temperature.